1. Field of the Invention
The present invention relates to an oil pump device for an engine, specifically, to the oil pump device including a mechanical oil pump to be driven by a crankshaft, and more specifically, to a variable-capacity oil pump.
2. Background Art
Conventionally, in an engine to be installed in an automobile, a mechanical oil pump to be driven by a crankshaft is used to supply, to each part of the engine, engine oil (hereinafter, simply called as oil) for lubricating or cooling a crankshaft, and bearing portions and sliding portions of a camshaft, or for operating a hydraulically operated device such as a VVT. The required amount of oil or the required hydraulic pressure differs depending on an operating state of the engine (such as an engine speed, a load, or a temperature). In view of the above, in a fixed-capacity oil pump, oil of a predetermined flow rate is discharged from the oil pump, and a relief valve provided in a discharge passage is controlled depending on an operating state of the engine to supply oil of a required amount to each part of the engine. However, oil of an amount exceeding the required amount is returned to an oil pan. Therefore, the work of the oil pump by the excessive amount of oil may be useless. This may deteriorate the fuel economy.
In view of the above, there is known a variable-capacity oil pump capable of changing the discharge amount, namely, the hydraulic pressure (discharge pressure) while being driven by a crankshaft. In the variable-capacity oil pump, it is possible to control discharge of oil by a required amount. This makes it possible to suppress useless work of the oil pump. For instance, Japanese Unexamined Patent Publication No. 2013-142297 (hereinafter, called as Patent Literature 1) discloses a technique in which supply of a hydraulic pressure to a decrease-side control pressure chamber for causing a pump casing of the variable capacity oil pump to swing toward the decrease side and to an increase-side control pressure chamber for causing the pump casing of the variable-capacity oil pump to swing toward the increase side is switched by an electrically operated control valve (electromagnetic spool valve) when the engine is operated in a low load operation mode and in a middle to high load operation mode so as to adjust the discharge amount of the oil pump to increase or decrease depending on an operating state of the engine.
In the technique disclosed in Patent Literature 1, when the engine is in a warm-up operation mode or in a low load operation mode after the engine is started, excitation current is supplied to the electrically operated control valve (energized state). Then, a hydraulic pressure is supplied to the decrease-side control pressure chamber, and the pump casing is caused to swing toward the decrease side. Thus, the discharge amount of the oil pump decreases. On the other hand, when the engine is operated in a middle to high load operation mode after a warm-up operation is completed, the supply of excitation current to the electrically operated control valve is stopped (non-energized state). Then, a hydraulic pressure is supplied to the increase-side control pressure chamber, and the pump casing is caused to swing toward the increase side. Thus, the discharge amount of the oil pump increases. The increase-side control pressure chamber is provided with a return spring for constantly urging the pump casing toward the increase side all the time including a time when a hydraulic pressure is not supplied to the increase-side control pressure chamber.
In the aforementioned configuration, electric power consumption may increase because excitation current is constantly supplied to the electrically operated control valve when the engine is operated in a low load operation mode, which is frequently used for the engine. In addition to the above, it is necessary to apply a force exceeding the urging force of the return spring in order to swing the pump casing toward the decrease side. In order to decrease the discharge amount of the oil pump with enhanced responsiveness, it is necessary to supply a relatively high hydraulic pressure to the decrease-side control pressure chamber. Particularly, the latter issue is serious because the hydraulic pressure tends to lower when oil of a low viscosity is used in order to improve the fuel economy.
As another example of the aforementioned variable-capacity oil pump, Japanese Unexamined Patent Publication No. 2014-51924 (hereinafter, called as Patent Literature 2) discloses an oil pump provided with, in addition to a first coil spring (return spring) for urging a pump casing toward the increase side, a second coil spring for urging the pump casing toward the decrease side.
However, in Patent Literature 2, the spring load (resilient force) of each of the first coil spring and the second coil spring is set to be such a value that the pump casing is urged to a maximum swing position (maximum eccentric position) on the increase side when the oil pump is stopped. In other words, the oil pump disclosed in Patent Literature 2 has basically the same configuration as the oil pump disclosed in Patent Literature 1. In Patent Literature 2, the second coil spring merely and temporarily assists movement of the pump casing in the initial stage when the pump casing is caused to swing toward the decrease side. Therefore, the oil pump disclosed in Patent Literature 2 also fails to solve the problem involved in Patent Literature 1.